1. Field of the Invention
The present invention relates to a hydrogen storage material capable of reversibly storing or releasing hydrogen and a method for producing the same.
2. Description of the Related Art
Fuel-cell vehicles are equipped with a fuel cell for generating an electric power utilizing an electrochemical reaction between hydrogen and oxygen. Thus, a motor of the fuel-cell vehicle is actuated by the electric power from the fuel cell to generate a driving force for rotating tires.
The oxygen can be obtained from the air, and the hydrogen is generally supplied from a hydrogen storage vessel. Therefore, the fuel-cell vehicle is further equipped with the hydrogen storage vessel.
As the hydrogen storage vessel has a higher hydrogen storage capacity, the fuel-cell vehicle can be driven over a longer distance. However, when the fuel-cell vehicle contains an excessively large gas storage vessel, the vehicle disadvantageously has an increased weight, resulting in a high load on the fuel cell. From this viewpoint, various methods have been studied for increasing the hydrogen storage capacity of the hydrogen storage vessel while preventing the volume increase. In one of the methods, a hydrogen storage material is placed inside the vessel. For example, AlH3, which can store hydrogen at a high ratio of approximately 10% by weight based on its own weight, is reported as an effective hydrogen storage material in Japanese Laid-Open Patent Publication No. 2004-018980 (particularly paragraphs [0060] to [0062]).
As shown in FIG. 17, a crystalline AlH3, has a microstructure containing approximately square-shaped matrix phases 2 and a grain boundary phase 3 disposed therebetween. In this case, the matrix phases 2 have a side length t1 of approximately 100 μm, and the grain boundary phase 3 has a width w1 of several micrometers and occupies only a several volume percent of the structure. In an X-ray diffraction measurement of the crystalline AlH3, a sharp peak of at least one of α, β, and γ phases is observed in the diffraction pattern.
It should be noted that the matrix phases 2 are composed of AlH3 having a crystal lattice containing Al and H, and the grain boundary phase 3 is composed of a solid solution of H in an amorphous Al.
In the crystalline AlH3 1, hydrogen is stored in accordance with the following formula (1), while the stored hydrogen is released in accordance with the formula (2). The formulae (1) and (2) represent reactions in an arbitrary storage/release site, and do not mean that all sites of the crystalline AlH3 1 are oxidized and reduced.Al+3/2H2→AlH3  (1)AlH3→Al+3/2H2  (2)
The reaction of the formula (2) can be relatively readily induced, but that of the formula (1) cannot be readily induced. As described in Japanese Laid-Open Patent Publication No. 2004-018980 (particularly paragraphs [0060] to [0062]), the hydrogen gas storage can be repeated only when the AlH3 is doped with Ti and NaH and then ball-milled under a hydrogen pressure of 100 atm.
In addition, as described in Sergei K. Konovalov and Boris M. Bulychev, Inorganic Chemistry, 1995, 34, 172-175 (particularly page 173, right column, lines 26-28 and FIG. 2), when the Al is hydrogenated by H2 gas contact in a gas-phase process, the contact has to be carried out under a high pressure of more than 2.5 GPa (about 25000 atm) at a temperature of 280° C. to 300° C. or under a further high pressure of 4 to 6 GPa at a temperature of 450° C. to 550° C.
As described above, the crystalline AlH3 is notably disadvantageous in that it cannot readily store the hydrogen.